Microwave apparatus



Nov. 22, 1955 y L. D. SMULLIN 2,724,305

MICROWAVE APPARATUS Filed D60. 2, 1950 2 Sheets-Sheet 1 Fig. I f

i. 5 i i T i T 7,

2 Fig. 2

Fig. 3

1 l l l INVENTOR LOUIS D. SMULLIN ATTORNEYS 2,724,805 MICROWAVE APPARATUS Louis D. Smullin, Watertown, Mass., assignor, by mesne assignments, to the United States of America Application December 2, 1950, Serial No. 193,815

7 Claims. (Cl. 333-73) This invention relates to methods and apparatus for introducing lumped reactance elements into microwave equipment, and more specifically to improvements in reactive irises in waveguides.

An object of the invention is to provide a waveguide iris which may be conveniently and accurately constructed with a gap width on the order of 0.005 of an inch, thus permitting the attainment of much higher reactances than has been conveniently obtained heretofore in waveguide irises.

A further object of this invention is to provide an iris which behaves electrically as if it were infinitely thin. A capacitive iris having this characteristic behaves as a pure shunt capacitance.

A further object of this invention is to provide a wave guide iris in which the gap width may be readily adjusted so as to change the magnitude of the iris reactance to meet a variety of coupling conditions.

The usual method of producing a lumped reactance in a waveguide is to introduce some kind of an obstacle in the waveguide so as to distort the electric and magnetic field conditions within the Waveguide. A distortion of the electric field is accompanied by a storage of energy in the electric field resulting in a capacitive reactance. A distortion of the magnetic field is accompanied by a storage of energy in the magnetic field resulting in an inductive reactance.

Such obstacles commonly take the form of irises which consist of thin metal walls placed across the inside and perpendicular to the walls of the Waveguide and are arranged so as to leave only a narrow gap through which energy may be propagated along the waveguide. Such irises are commonly of three forms. A capacitive iris has its gap arranged parallel to the magnetic field in the waveguide so that only the electric field is distorted at the iris. An inductive iris has its gap arranged parallel to the electric field in the waveguide so that only the magnetic field is distorted. In both the inductive and capacitive irises the gap extends the entire width of the waveguide wall. The third form of iris is produced by placing a rectangular slit in the iris wall which does not extend the entire width of the waveguide. Such a configuration distorts both the electric and magnetic fields and results in a combination of the inductive and capacitive irises to produce a resonant iris.

These prior forms of waveguide iris have the disadvantage that they are difficult to construct to close tolerances. This is particularly true of the capacitive form of the iris where gap widths of the order of 0.01 of an inch are necessary to produce appreciable reactance. Difiiculties with warping are encountered in the resonant form of iris.

A further disadvantage with these forms of iris is that they are inherently not readily adaptable to adjustment of the gap width of a given iris. These and other disadvantages have considerably limited the use of irises in microwave equipment.

With the foregoing objects in view, the present invenrrnte States, Patent ice tion contemplates the provision of waveguide reactances which have the electrical equivalent of an ideal iris while avoiding the major mechanical disadvantages of existing irises. In the best form now known the invention provides two waveguide sections at a junction the sections being laterally displaced with respect to each other. The displacement is made in a plane generally parallel to the electric and magnetic field vectors in the waveguide. This displacement places an obstacle in the waveguide which acts to distort the electric of magnetic field of a wave in the guide. If the displacement has a magnitude slightly smaller than the inside dimension of the waveguide at the junction, then a narrow gap may be readily formed, which behaves electrically in much the same manner as a thin wall type of iris gap. Thus if the displacement is parallel to the magnetic field in the waveguide, only the magnetic field is distorted and an inductive iris is produced. If the displacement is parallel to the electric field, only the electric field is distorted and a capacitive iris is produced. A displacement along some intermediate line distorts both the electric and magnetic fields and a resonant iris is produced.

Referring now to the drawings in which a preferred embodiment of the invention is shown: Fig. 1 shows a section along the central axis of a rectangular waveguide parallel to the E vector in the waveguide; Fig. 2 shows a cross-section through the waveguide at line 22 showing the displacement of two waveguide sections in a capacitive iris; Fig. 3 is a cross-section showing the displacement of two Waveguide sections in an inductive iris; Fig. 4 is a cross-section showing the displacement of two waveguide sections in a resonant iris; Fig. 5 is a view showing the arrangement of waveguide sections to form an iris at an angular junction; Fig. 6 is a view showing a number of iris junctions together with suitable waveguide sections to form a microwave filter assembly; Fig. 7 is a schematic diagram showing the hyperbolic tuning curves for a resonant iris for various values of wavelength and gap dimensions.

One embodiment of the invention is shown in Figs. 1 and 2. A rectangular waveguide section 1 of conventional form is adapted to propagate a wave according to well-known principles. A second rectangular waveguide section 3 joins the section 1 but is displaced therefrom by a'distance D, which is indicated in these figures as being parallel to the electric field vector E. The sections 1 and 3 have flanges 4 and 5, respectively, rigidly attached at their ends adjacent to the junction. Each flange preferably is of large enough dimension so that even the maximum displacement, that is, when D is equal to the inside dimension of the waveguide, allows the flanges to overlap by an amount sufficient to provide a satisfactory joint. The flange and end construction of the waveguide should be smooth enough to provide a tight fit at the junction of the two sections.

The waveguide sections may be held rigidly together at the junction by any suitable means. If the gap distance is not to be changed in a given installation it is preferable to solder, weld, or bolt the flanges together. If it is necessary to change the gap distance from time to time, the sections may be held together by a suitable clamp 6. A feeler gauge may be used to set the gap distance before clamping or soldering.

The foregoing is a capacitive iris in that a displacement of the waveguide sections 3 and 1 with respect to each other produces a distortion of the electric field while leaving the magnetic field substantially undistorted. This results from the fact that the displacement is made parallel to the electric vector E, as shown in Figs. 1 and 2. The resulting iris gap behaves electrically as if it were infinitely thin and follows the theoretically derived ex- -tain a constant condition of resonance.

pression for the normalized susceptance of an infinitely thin. asymmetrical gap,

where B is the actual susceptance, Yo is the characteristic admittance, G is the gap distance, h is the waveguide height, A is the wavelength of the radiation in the waveguide, and In is the Naperian base logarithm.

An inductive iris junction is attained as shown in Fig. 3 wherein the displacement of the waveguide sections is made along a line perpendicular to the electric vector E in the waveguide. A displacement so made distorts only the magnetic field in the waveguide leaving the electric field substantially undistorted. Because of the asymmetry of the magnetic field on either side of the junction the equivalent circuit must be described as a T network of inductances. The susceptance of the inductive shunt element thus produced follows very closely the theoretically derived expression for an infinitely thin asymmetrical gap expressed in the relation for normalized susceptance by the equation,

where, again, B is the actual susceptance of the iris, Yo is the characteristic admittance of the waveguide, G is the gap distance, a is the width of the waveguide, and is the wavelength of the radiation in the waveguide.

An important feature of the present invention is that a displacement of the waveguide sections having components both parallel to and perpendicular to the electric vector E produces a combined inductive and capacitive effect which results in a resonant iris junction. This displacement is illustrated in Fig. 4. The resonant iris junction is conveniently described by its tuning curves shown in Fig. 7. These curves describe hyperbolic paths, there being one curve for each different wavelength of radiation in the waveguide, along which one waveguide section may be moved with respect to the other in order to main- That is, for a given wavelength the position of one waveguide section with respect to the other may be at any point along the hyperbola corresponding to that wavelength in order for the resonant condition to be maintained. The Q of the resonant iris increases as the gap dimensions decrease. For a different wavelength a different hyperbolic path is necessary to maintain resonance as shown in Fig. 7. The mathematical expression for these hyperbolas defining the resonant condition of the iris is given by the equation:

where a and h are the width and height of the guide respectively, a and h are the width and height of the rectangular gap, respectivel and Ag is the guide wavelength.

It is to be understood that the present invention is by no means limited to the three forms illustrated in Figs. 1, 2, 3 and 4 and described above. In these forms the displacement of the two waveguide sections was shown as being in a plane perpendicular to the central axis of the waveguide. More generally any displacement in a plane which passes through the waveguide, with suitable flanges to implement such a displacement and hold the sections together, will produce distortion in the electric or magnetic field and would thus give rise to a lumped reactance of a capacitive, inductive or resonant nature. Nor is it necessary that the central axes of the two abutting waveguide sections be parallel to each other. A more general form of a waveguide iris junction is shown in Fig. 5 where the displacement is made laterally and the axes of the waveguide sections are not parallel to each other. The term flaterally as used here and also in the claims to follow, is intended to mean in a plane intersecting the junction of the central axes of the waveguide sections when such displacement is zero.

The more general form of the iris junction, as shown in Fig. 5, comprises a rectangular waveguide section 10 of conventional form adapted to propagate a wave according to well known principles and a second rectangular waveguide section 11 joining the section 10 but displaced laterally therefrom by a distance D. The sections lit) and 11 are provided with flanges 12 and 13, respectively, so as to form a physical embodiment of the plane in which the displacement is to be maintained. The sections are held in place by any suitable means as described above. In the form shown in Fig. 5 the gap extends perpendicular to the direction of the electric vector E in the waveguide. Due to the mismatch encountered in an angular waveguide junction of the type shown and due to the complex geometry of the electric and magnetic field in such a junction as is shown, it is not possible to express the susceptance of this junction by the same expressions that are written above. Fora given angle between the two sections at the junction, however, the mismatch may be partially or wholly compensated for by the impedance introduced at the waveguide iris junction.

One of the common problems to which the present invention may be applied is the design and construction of suitable filters for microwave assemblies. Such a filter assembly is shown in Fig. 6 which discloses a number of suitable waveguide sections joined according to the principles of the present invention. Each junction shown represents a lumped reactance, either capacitive, inductive, or resonant, each individually capable of adjustment so as toproduce a desired overall filter characteristic.

The above description has dealt with the principal features of the present invention applied to junctions formed by rectangular waveguide sections. It is not necessary that the waveguide sections be rectangular in order to .produce a waveguide reactance. Thus, appropriate displacement of other shapes of waveguide sections, for example, circular Waveguide sections, would produce a lumped reactance at the junction according to the principles of the present invention.

Having fully described the general features of my invention and certain particular embodiments of my invention, I claim:

1. A waveguide system comprising a plurality of rectangular waveguide elements having plane ends disposed in abutting relation, abutting flanges disposed outwardly of the walls of the elements in coplanar relation to the plane ends of the elements, the interiors of the elements having substantially constant cross-section, and means for securing the Waveguide elements with their abutting ends in closely fitting laterally offset relation to provide a lumped reactance at the junction of said elements, the amount of offset being such that the area of the common opening at the junction between the two elements is only a minor portion of the crosssectional area of either of said waveguide elements.

2. A waveguide system comprising a plurality of rectangular waveguide elements having plane ends disposed in abutting relation, abutting flanges disposed outwardly of the walls of the elements in coplanar relation to the plane ends of said elements, the cross-sections of the waveguides being substantially uniform, and means for securing the waveguide elements with their abutting ends in closely fitting laterally oitset relation to provide a rectangular iris, the amount of oflset being such that the area of the common opening at the junction between the two elements is only a minor portion of the cross-sectional area of either of said waveguide elements.

3. A waveguide system comprising a plurality of rectangular waveguide elements having plane ends disposed inabutting relation, abutting flanges disposed outwardly of the walls of the elements in coplanar relation to the plane ends of said elements, the interiors of the waveguide elements having substantially uniform cross-section, and means for securing the waveguide elements with their abutting ends in close fitting contact and offset in a direction normal to the walls of the waveguide elements to provide a rectangular slot equivalent to an infinitely thin iris and constituting a lumped reactance at the junction of said elements, the area of said slot representing a minor fraction of the cross-sectional area of either of said waveguide elements.

4. A waveguide junction comprising two rectangular waveguide sections having plane ends disposed in abutting relation, said ends having outwardly directed flanges coplanar with the plane ends of said elements, said waveguide sections having substantially constant cross-section, said waveguide elements being secured with the flanges in close fitting contact and with the elements oifset along axes normal to the walls of the elements to define a rectangular aperture smaller in both dimensions than the cross-section of the waveguide sections to provide a resonant iris junction, wherein the iris dimensions are determined from the hyperbola defined by where a and 11 are respectively the width and height of 6 the guide, a and h are the width and height of the rectangular gap, and A is the wavelength of the radiation in the waveguide.

5. A waveguide system as in claim 1 having a plurality of junctions with at least one of the junctions being ottset in a direction normal to the electric field and at least another junction being oflset in a direction normal to the magnetic field, to provide a waveguide filter having lumped capacitive and inductive reactances.

6. A waveguide system as in claim 2 wherein said iris is a narrow gap parallel to the magnetic field in the waveguides whereby a capacitive effect is produced.

7. A waveguide system as in claim 2 wherein said iris is a narrow gap parallel to the electric field in the waveguides whereby an inductive effect is produced.

References Cited in the file of this patent UNITED STATES PATENTS 2,423,130 Tyrrell July 1, 1947 2,496,772 Bradley Feb. 7, 1950 2,541,375 Mumford Feb. 13, 1951 2,579,327 Lund Dec. 18, 1951 2,597,143 Aron May 20, 1952 

